THE DIVING BEHAVIOUR OF sharks appears to be influenced by the moon, water temperature and time of day, researchers have revealed.

A study of about 40 grey reef sharks, commonly found on coral reefs in northern Australia and the Indo-Pacific, found they stayed in deep water during a full moon and moved to shallow water with the new moon.

"To our knowledge, this is the first time such patterns have been observed in detail for reef sharks," says lead researcher Gabriel Vianna, from the University of Western Australia (UWA) in Perth.

Sharks affected by moon

The sharks were tagged near Palau, east of the Philippines, and followed for two years. During this time, scientists from UWA and the Australian Institute of Marine Science recorded their movement and diving patterns.

The findings, published this week in the journal PLOS ONE, reveal that sharks descended to greater depths, and used a wider range of depths, around the time of the full moon.

Diving was also affected by seasonal changes, as the group, which mostly consisted of adult females, was recorded diving to an average depth of 35m in winter and 60m in spring.

In winter, the sharks remained closer to the surface, where the water was warmer. During summer, however, the sharks moved to a range of depths.

The researchers suggest that because sharks are cold blooded, they may prefer warmer water to conserve their energy. Warm water may also provide optimal conditions for foraging for food, the study says.

Sharks dive deeper in the morning

The findings also suggest that the time of day could affect how deeply sharks dive.

"We were surprised to see sharks going progressively deeper during the morning and the exact inverse pattern in the afternoon, gradually rising towards the surface," says Gabriel, adding that the behaviour may relate to how much light is reflected on the reef at different times during the day.

Better knowledge of shark behaviour could help reduce the risk of sharks coming into contact with locals and tourists fishing, particularly if their diving behaviour can be predicted at certain times of the day.

"In places such as Palau, which relies heavily on marine tourism and where sharks are a major tourist attraction, the fishing of a few dozen sharks from popular dive sites could have a very negative impact on the national economy," Gabriel says.